1. Field of the Invention
The present invention relates to a method for repairing a pattern using laser, including photomask repairing processing and fine pattern defect repairing processing, and a laser-based pattern repair apparatus.
The present application claims priority of Japanese Patent Application No. 2000-222556 filed on Jul. 24, 2000, which is hereby incorporated by reference.
2. Description of the Related Art
A laser processing apparatus employing laser oscillation light, precision movable stage, and scanning technology in combination is used widely in various areas. In particular, An apparatus for repairing an ultra-fine pattern defect by using pulsed laser light, is recently used not only in semiconductor technology, as a laser mask repair apparatus or a like but also in medical field. In processes of photolithography being a main process of LSI (Large Scale Integrated circuit) manufacturing, a plurality of photomasks is used. The laser mask repair apparatus is an apparatus adapted to repair a variety of defects on the photomask using a laser. As shown in a cross-sectional view in FIG. 9A, the photomask is so configured that a pattern made up of a highly light-tight metal chromium thin layer is formed on a main face of silica-based glass having high optical transmittance of light with a short wavelength. A variety of types of defects on the pattern are shown in FIG. 9B. The pattern defects can be broadly classified into two types, one being a residual defect generally called an opaque defect and the other being a chip generally called a clear defect. The opaque defect is repaired by laser-based removing processing or a like and the clear defect is repaired by laser CVD (Chemical Vapor Deposition) processing. The laser-based removing processing is a most general photomask defect repair processing performed by using the laser. Most of laser repair apparatuses are based on the above laser-based removing processing technologies. The laser used in the laser repair apparatus is a solid-state laser employing Nd:YAG (Neodymium:YttriumAluminumGarnet), Nd:YLF (Neodymium:Yttrium Lithium Fluoride), Nd:YVO4 (Neodymium:Vanadium tetroxide Yttrium) or a like as a laser medium. The reason that such solid-state lasers are used is firstly because Q-switched pulse oscillation enabling high peak power (short pulse width) to be provided can be achieved in a stable manner, secondly because it can produce harmonics, and thirdly because it can be small-sized and is excellent in controllability and/or maintainability.
Processing in which laser light is condensed on a microspot to cause a chromium thin layer to be evaporated and removed is called a zapping process. In the zapping processing, Q-switched pulsed laser light having a pulse width of about 5 nanoseconds to 10 nanoseconds is conventionally used.
To enhance controllability in the zapping processing, when the pulse width of the laser is shortened, a method is used in which a length of an optical resonator is made smaller, intensity of light for pumping to be applied to the laser medium is made higher to produce the Q-switched laser pulse that can provide a high-speed rise. For example, conventionally, by a MOPA (Master Oscillator Power Amplifier) method in which an LD (Laser Diode) pumping type Nd:YVO4 microchip laser is used as a main oscillator (master oscillator) in the optical resonator having a resonator length of about 25 mm and laser light emitted from the oscillator is amplified by an optical amplifier, sub-nanosecond pulsed laser light with a pulse width of 0.85 ns (nanoseconds) that provides energy of 10 mJ/pulse is produced and is used as a pumping source (Y. Kyusho et al.; OSA TOPS on Advanced Solid-State Lasers, Vol. 1 (1996)).
However, the conventional laser photomask repair technology has a problem in that phenomena as shown in FIG. 10 occur at a time of the zapping processing. That is, swelling caused by heat affected zones, limpness of an edge, splashes, and a decrease in mask transmittance caused by damage to a glass substrate occur when the zapping is performed on the Cr (chromium) layer.
A first problem is that, when pulsed laser light having a pulse width of 5 ns to 10 ns is used, the heat affected zone having a length of about 0.5 μm to 1.0 μm is produced. A length of thermal diffusion of Cr used in the photomask is 0.7 μm when the laser pulse width is 5 ns and about 1.4 μm when the pulse width is 10 ns and heat reaches within the length of thermal diffusion. In the laser pulse having such the long width as above, in principle, the heat affected zone occurs more or less. In some cases, the heat affected zone causes the limpness of the edge that decreases fabrication accuracy and a part of the heat affected zone flies about in a form of as plash and deposits on the pattern, thus causing the pattern defect. To avoid this, a method has been conventionally employed to prevent the heat affected zone being fused by properly selecting laser power. Even when sub-nano second pulsed laser light having a pulse width of 0.85 ns, the heat affected zone of 0.12 μm in length is mathematically produced. In order to reduce the length of the heat affected zone to not more than 0.01 μm, pulsed laser light having a pulse width of about 70 ps (picoseconds) is required mathematically. However, it is impossible for a conventional Q-switched laser to produce the pulsed laser light having a pulse width being within this range of 70 ps.
A second problem is that, when the peak power of the laser pulse continues to be increased, an edge portion of a laser irradiation section swells vertically. That is, if the pulse width becomes shorter while the pulse energy in one shot pulse is at a same level, the peak power increases, however, if only the peak power is increased while an area affected by heat is decreased, photon pressure acts, when the laser is applied, as counteractive force in a direction opposite to a light incident direction and, as a result, a very large burr-shaped swelling occurs in a fused layer between the laser irradiation section and laser non-irradiation section. When the pulse width is several 10 ps, in some cases, the burr with a length of not less than 1 μm is produced vertically. This type of burr causes rolling-up and/or damage to the pattern, which should be avoided. In some cases, the fused parts splash, depending on the laser power density, not only in the vertical direction but also in the horizontal direction relative to the photo mask. When the conventional pulsed laser light having a pulse width of the order of nanoseconds is used, the splash having a length of 0.5 μm to 1 μm is produced in many cases. This causes new pattern defects to occur around the laser irradiation section.
A third problem is that the occurrence of the splash is influenced by a relation between a size of an area to which the laser is applied and an irradiated laser power density. That is, a shape of the defect or the state of occurrence of the splash is changed depending on a ratio of a longer side to a shorter side of a rectangular, thus producing an unstable state on the pattern.
A fourth problem is that the employed laser light causes damage to the glass substrate. If power of the laser light is increased or a wavelength of the laser light to be applied is changed to be that of ultraviolet light, damage to the glass substrate occurs to one degree or another. This damage to the glass substrate presents a serious problem in terms of quality of the photomask because the damage causes the transmittance to decrease at a time of exposure processing or an interference pattern to occur.